In recent years, there has been increasing interest in energy storage technologies. As the application fields of energy storage technologies have been extended to mobile phones, camcorders, notebook computers and even electric cars, there has been a growing demand for high energy-density batteries as power sources for such electronic devices. In response to this demand, research on lithium secondary batteries is being actively undertaken.
Many companies have produced a variety of lithium secondary batteries with different safety characteristics. It is very important to evaluate and ensure safety of such lithium secondary batteries. The most important consideration for safety is that operational failure or malfunction of lithium secondary batteries should not cause injury to users. For this purpose, safety regulations strictly restrict the possibilities of dangers (such as fire and smoke) of lithium secondary batteries. In connection with the safety characteristics of a lithium secondary battery including a separator, overheating of the lithium secondary battery may cause thermal runaway or puncture of the separator may pose an increased risk of explosion. In particular, a porous polyolefin substrate commonly used as a separator of a lithium secondary battery undergoes extreme thermal shrinkage at a temperature of 100° C. or higher due to its material characteristics and production processes including elongation. This thermal shrinkage behavior may cause short circuits between a cathode and an anode.
Various proposals have been made to solve the above safety problems of lithium secondary batteries. For example, Korean Unexamined Patent Publication Nos. 10-2006-72065 and 10-2007-231 disclose organic-inorganic composite separators, each of which includes a porous substrate and a porous coating layer formed by coating a mixture of inorganic particles and a binder polymer on at least one surface of the porous substrate. The inorganic particles present in the porous coating layer coated on the porous substrate serve as spacers that can maintain a physical shape of the porous coating layer to inhibit the porous substrate from thermal shrinkage when an electrochemical device overheats. Interstitial volumes present between the inorganic particles form fine pores of the porous coating layer.
As the charge/discharge cycles of a battery proceed, metal ions (usually transition metal ions) other than lithium are also dissolved from a lithium-containing metal oxide as a cathode active material and cause decomposition of an electrolyte solution. Along with the recent increasing demand for high-capacity lithium secondary batteries, high voltages are required as conditions for battery operation. However, high-voltage operating conditions primarily increase side reactions of electrolyte solutions and also induce rapid dissolution of transition metals from lithium transition metal oxides as typical cathode active materials. The dissolved transition metals further promote side reactions of electrolyte solutions.
Moreover, it was found that side reactions of electrolyte solutions occur more actively when the conventional organic-inorganic composite separators are used under high-voltage operating conditions than when general porous polyolefin separators are used under the same conditions.